Method for generating charged particles

ABSTRACT

A method for establishing a calibrating standard for wafer inspection includes depositing solid ionized particles of a known size range with an aerosol onto a wafer. The method also includes depositing particles onto a wafer in a deposition chamber by using an aerosol stream and the solid particles suspended in a gas; ionizing the aerosol stream with a negative or positive charge polarity or both by passing the aerosol stream through a non-radioactive ionizer to produce charged particles and supplying such aerosol stream to the deposition chamber.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of and claims priority of U.S.patent application Ser. No. 12/985,105, filed Jan. 5, 2011, which is adivisional of and claims priority of U.S. patent application Ser. No.11/252,252, filed Oct. 17, 2005, which claims the benefit of U.S.provisional patent application Ser. No. 60/619,734, filed Oct. 18, 2004,the contents of all of the above which are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

This invention relates to methods and apparatus for generating chargedparticles for size measurement by electric mobility and deposition on awafer and in particular relates to particle deposition for waferinspection, surface cleaning and as seed nuclei in semiconductor devicefabrication.

Solid particles of an accurately known particle size deposited on awafer are useful as standards of calibration for wafer surfaceinspecting equipment. Solid particles deposited on a wafer can also beused as artificial contaminants for testing wafer cleaning tools todetermine their efficiency for particle removal from the wafer surface.In addition, particles deposited on a wafer can be used as seed nucleifor subsequent processing to form unique thin films with desiredphysical or chemical properties.

Aerosols containing solid or liquid particles suspended in a gas mediumare useful for a variety of purposes. When the aerosol particles areelectrically charged, the particle size can be measured by measuring itselectrical mobility in the gas medium. Charged particles in an aerosolcan also be deposited on a semiconductor wafer as artificialcontaminants for a variety of purposes.

One application relates to wafer surface inspection by laser lightscattering, electron microscopy and other methods for detecting thepresence of particles on the wafer surface and for measuring theparticle size. Calibration of such surface inspecting tools requiresdepositing solid particles of an accurately known particle size on thewafer for use as calibration standards. For such applications,polystyrene latex (PSL) spheres are usually used. The PSL spheres aregenerally dispersed in an aqueous medium and atomized by a pressurizedgas source to form droplets. The droplets are then evaporated to allowthe individual PSL spheres to appear as solid, spherical particles ofPSL suspended in the atomizing gas, thus forming a PSL aerosol. Aerosolrefers to a gas containing suspended particles. PSL aerosol, therefore,refers to an aerosol in which the suspended particles are polystyrenelatex spheres. The PSL spheres are then deposited from the aerosol ontothe wafer surface to produce standard wafers for calibration purposes.

Deposition of small particles from an aerosol by the usual mechanisms ofgravitational settling or diffusion is generally too slow and notsuitable for practical applications. For most applications, the rate ofdeposition needs to be increased. This can be accomplished by usingcharged particles in combination with an electric field to causeincreased rate of deposition by the application of an electrical forceon the charged particles.

Aerosols produced by atomization usually are not highly charged. Acommon method to increase the particle charge is to expose the aerosolto a source of ionizing radiation from a radioactive material. The highenergy nuclear particles of alpha, beta and gamma rays emitted bymaterial undergoing radioactive decay ionize the molecules of the gas toform molecular ions of both a positive and a negative polarity. Thesemolecular ions then collide with the aerosol particles suspended in thegas to cause the particles to become charged. The resulting particlecharge is usually bipolar, meaning that some particles are positivelycharged, and some are negatively charge. Since roughly equalconcentrations of positively and negatively charged particles arecreated, the aerosol remains substantially neutral even though theindividual particles are charged. As a result, exposing an aerosol to asource of ionizing radiation from a radioactive material is oftenreferred to as a neutralization process even though the end result alsoincludes the production of charged particles of both a positive and anegative electrical polarity.

The most common radioactive material used for aerosol neutralizationincludes polonium 210 and krypton 85. Both of these materials are widelyused. Polonium 210 is an alpha emitter with a half life of 138 daysthrough radioactive decay, while krypton 85 is a beta emitter with ahalf life of 10.3 years. The use of radioactive ionizers for aerosolneutralization and aerosol particle charging are described in References1 and 2.

Because of health, environmental and security concerns, radioactivematerials for research, commercial or industrial use are generallyregulated by appropriate governmental agencies. These regulations arebecoming increasingly more stringent making the use of a radioactiveionizer a less desirable method for gas ionization and particle chargingfor wafer deposition and other applications. A non-radioactivealternative is therefore needed.

Another application relates to the generation and deposition of solidparticles on a wafer for use as artificial contaminants for wafercleaning studies. For such applications, the particles are generallydeposited on a wafer. The wafer is then scanned by a scanning surfaceinspecting tool to determine the number of particles deposited. Thewafer is then subjected to cleaning by the wafer cleaning tool.Following cleaning, the wafer is scanned again to provide a new particlecount. The difference in the initial and final particle count is thenumber of particles removed by the wafer cleaning tool. The percent ofparticle removal is then referred to as a cleaning efficiency. Usingwafers artificially contaminated by particles, the particle removalefficiency of cleaning tools can be easily measured.

For wafer cleaning studies, various particle materials need to be used.Particle materials of the greatest interest include silicon, silicondioxide, silicon nitride, tungsten, and copper, among others. Dry solidparticles of a variety of materials and sizes, therefore, need to bedeposited on a wafer to produce test wafers for wafer cleaning studies.Since different particle materials have different adhesion forcecharacteristics when deposited on the wafer surface, it is importantthat the material of particles used for testing the wafer cleaning toolbe similar to the material of real contaminant particles found on thewafer.

Another application is the generation and deposition of solid or liquidparticles on a wafer to serve as seed nuclei for subsequent waferprocessing by chemical vapor deposition, atomic layer deposition, andother thin-film deposition processes for semiconductor integratedcircuit device fabrication. Formation of thin film by various filmformation processes is facilitated by the presence of seed nuclei forfilm formation and growth. For such applications, dry solid particles ofthe desired material can be deposited on a wafer. Alternatively, smallliquid particles can be deposited on the wafer which can then be reactedchemically with another material or thermally processed to produce thedesired solid seed nuclei for such applications.

In all of these applications, the number of particles deposited on thewafer is generally quite small, when compared to the number of particlesneeded to cover the wafer surface completely. As such, this applicationdiffers from other methods of droplet deposition for thin filmfabrication such as those described in U.S. Pat. No. 5,316,579. Fordepositing droplets to form thin films, the number of droplets depositedmust be sufficiently large to provide complete surface coverage toproduce a continuous thin film on the surface for subsequent processingto form a solid thin film with the desired physical and/or chemicalproperties. For the present application, the number of particlesdeposited is small and the deposited particles remain as discreteentities on the wafer surface rather than as a continuous film on thewafer surface.

SUMMARY OF THE INVENTION

The present invention includes a method and apparatus for generatingcharged aerosol particles for size measurement by electrical mobilityand deposition on a wafer. The method involves preparing an aqueoussuspension of the solid particles and atomizing the liquid by acompressed gas atomizer to form a droplet aerosol. Followingatomization, the droplets are evaporated to form an aerosol containingsuspended solid particles in the gas. A non-radioactive ionizer is thenused to increase gaseous ions in the aerosol for charging the particles.The non-radioactive ionizer is comprised of a high voltage coronadischarge electrode connected to a source of DC or AC voltage. Thecorona discharge produces gaseous ions, which in turn collide with theparticles to cause the particles to become electrically charged. Inanother aspect of the method, fine droplets containing suspended solidparticles are atomized by an electrospray droplet generator. Thedroplets are then evaporated to form an aerosol comprised of solidparticles suspended in a gas. The aerosol is then mixed with gaseousions produced by a corona discharge to produce a suitable level ofcharge on the particles for subsequent size-classification by electricalmobility and deposition on a wafer. The present invention also includesa method of establishing a calibrating standard for wafer inspection bydepositing solid ionized particles with an aerosol onto a wafer whereinthe solid particles are of a known size range.

The present invention also includes a charged particle generatingapparatus having an atomizer and a non-radioactive ionizer in fluidcommunication with each other. The present invention also includes acharged particle generating apparatus that includes an electrospraydroplet generator that produces an aerosol containing dry-solidparticles and a non-radioactive ionizer in fluid communication with eachother. The present invention also includes an apparatus for producing acharged monodispersed aerosol that includes a charged particle generatorcontaining a non-radioactive ionizer, and electrostatic classifier.

The present invention also includes an apparatus for providing chargedparticles for deposition on a substrate wherein the apparatus comprisesa generator for generating an aerosol with particles suspended in thegas and a non-radioactive ionizer that ionizes the gas in the aerosol toprovide ions of a positive or negative charge polarity or both in thegas causing the aerosol particles to be charged in a fashion suitablefor deposition on a substrate. The apparatus may also include anelectrospray droplet generator producing an aerosol with dry-solidparticles suspended in a gas; an ionizer that ionizes the molecules ofthe gas to provide ions of a positive or a negative charge or both inthe gas; and a mechanism that mixes the ionized gas with the aerosol toprovide charged particles in a fashion suitable for deposition on thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a charged particle generating apparatuscomprised of a compressed gas atomizer and a high voltage coronaionizer.

FIG. 2 is a schematic view of a charged particle generating apparatuscomprised of an electrospray droplet generator and a high voltage coronaionizer placed in the same housing.

FIG. 3 is a schematic view of a charged particle generator combined withan electrostatic classifier and a deposition chamber for particledeposition on a wafer surface.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of an apparatus for generating chargedparticles by the process of atomization combined with charging by ahigh-voltage corona ionizer. An atomizer, shown generally at 10,includes a reservoir 12 containing a liquid 22 to be atomized. A gasflow passageway 14 allows a compressed gas to be introduced through theinlet 16 to a restricting orifice 18 to form a high velocity gas jet.The high velocity gas jet causes suction, i.e. a vacuum, to be generatedin the exit gas flow passageway 20. As a result, liquid in the reservoir12 is aspirated into this low pressure, vacuum region. In this region,the high velocity gas jet causes the liquid 22 to be atomized to formdroplets. This droplet aerosol then flows out of the gas flow passageway20 into the space 21 above the liquid 22 in the reservoir 12.

To the right of the atomizer is a gas-flow passageway 30 with an inlet24 on the left and an outlet 34 on the right. Between the inlet 24 andthe outlet 34 is a second gas inlet 32 through which additional gas canbe introduced to mix with the aerosol and facilitate dropletevaporation. When water is atomized, a dry gas is introduced throughinlet 32 in order to lower the relative humidity of the mixture flowingout of the outlet 34. By this means the droplets are evaporated beforeentering the charging chamber containing the high-voltage coronaionizer.

To produce an aerosol comprised of dry-solid particles suspended in agas the particles, such as polystyrene latex (PSL) spheres, are firstdispersed in water, which is then placed in the reservoir 12 foratomization. The atomized droplets are then evaporated to leave behindsolid PSL spheres suspended in the gas to form a PSL aerosol.

As mentioned earlier, solid and/or liquid particles produced byatomization usually are not highly charged. The charge is generally toolow for most applications in which particles are deposited from anaerosol onto a surface. To produce a higher level of particle charge,the non-radioactive ionizer 40 is used. The non-radioactive ionizer 40includes a housing 42 with an internal cavity 44. The housing 40 alsoincludes a metal electrode 50 surrounded by electrical insulation 54,which is in turn surrounded by metal 56 as a shield. The electrode 50 isconnected to a source of high voltage (not shown). The insulation 54minimizes current leakage that may otherwise occur from the high voltageelectrode 52 to the housing 42 which is grounded 43.

The metal electrode 50 has a sharp tip 52 on its end. When a highvoltage on the order of a few thousand volts is applied to theelectrode, a high electric field gradient is established in theimmediate vicinity of the tip 52 causing the gas to break-down toproduce molecular ions by the process known as corona discharge. Theions can be unipolar, i.e. all of the same polarity, if the appliedvoltage is DC. If an AC voltage is applied, bipolar ions are produced.For purposes of generating charged aerosol particles for deposition on awafer, either a DC or an AC voltage can be used. The resulting aerosolparticle charge can be unipolar, i.e. all of the same electricalpolarity; or bipolar with some particles carrying a positive polarity ofcharge, and some a negative polarity.

Another method of producing charged particles for subsequent depositionon a wafer is to use a process known as electrospray. The electrospraydroplet generation process is widely used to generate molecular ions forElectrospray Ionization Mass Spectrometry, P. Kebarle and Yeunghaw Ho.,“On the Mechanism of Electrospray Mass Spectrometry”, Chapter 1,Electrospray Ionization Mass Spectrometry, Richard B. Cole (Ed.), pp.3-63, John Wiley and Sons (1997) (which is herein incorporated byreference in its entirety). Electrospray involves spraying a liquidthrough a fine capillary held at a high voltage to generate highlycharged droplets, which are then evaporated to produce molecular ions asan ion source for mass spectrometry. In the present invention, theelectrospray is used as a fine droplet generator to generate particlesfor deposition on a wafer.

In FIG. 2, the electrospray charged droplet generating apparatus isshown generally at 100. The apparatus has a housing 102 with twointerior cavities 104 and 130. The housing is provided with two gasinlets 106 and 116 and a gas outlet 109.

The electrospray droplet generating apparatus itself is shown generallyat 110. the apparatus 110 includes a fine capillary 112 surrounded byelectrical insulation 114 which in turn is surrounded by a metal tube116. The insulation 114 is spaced apart from tube 116 to provide anannular space 118 in between. The capillary 112 has an inlet 120 toallow a liquid to be introduced and an outlet 122 through which theliquid can exit. Both the liquid and the capillary are connected to asource of high voltage 160. The liquid source is shown as 165, whichcontains solid particles, such as PSL spheres, suspended in a liquid,such as water. As the liquid enters the inlet 120, the liquid encountersa high electric field gradient in the immediate vicinity of thecapillary. This high field gradient causes the liquid to accelerate. Asthe liquid velocity increases, the cross-sectional area of flow willdecrease. As a result, the liquid emerging from the capillary will forma conical shaped column with a large cross-sectional area at the baseand tapering to a fine tip at the top. This conical shaped liquid columnis commonly referred to as a Taylor cone. At the tip of the Taylor cone,the liquid is sprayed out to produce highly charged and very finedroplets in the electrospray chamber 130.

Since the electrospray will only operate properly if the gas surroundingthe Taylor cone does not break down to produce a corona discharge, agas, such as CO₂, that does not breakdown easily can be introducedthrough the annular gap space 118 between the insulation and the outertube and fill the chamber 130 with CO₂. Other gases that are not-easilyionizable, such as argon, helium, etc. can also be used. The gas sourceis shown at 170, which is connected to the annular space through theinlet 116.

A voltage ionizer 150 similar to the one described with reference toFIG. 1 is located upstream of the electrospray atomizer 110. The highvoltage ionizer 150 includes a metal electrode 152 with a sharp tip 154on one end. The electrode 152 is surrounded by electrical insulation156, which is in turn surrounded by a metal sheath 158. The electrode152 is connected to a source of high voltage 175, which can be either DCor AC. When DC high voltage is used, the polarity of the voltage must beopposite to the polarity of the voltage applied to the electrospraycapillary 120 so that gaseous ions generated by the high voltage willhave a polarity that is opposite to that of the charged dropletsproduced by the electrospray. The high-voltage ionizer includes theinterior cavity 104 in which the gaseous ions are produced. The cavity104 is connected to a gas source 180 supplying a gas flow through theinlet 106 to the cavity 104. This gas flow then carries the gaseous ionsand flow through the tube 108 where the gas is mixed with theelectrosprayed particles flowing through the orifice 132 from thechamber 130. The electrospray chamber 130 is supplied with a source ofdry gas 170 so that the droplets are evaporated in the chamber 130 toproduce an aerosol containing dry solid particles suspended in the gas.By this means the particle charge is quickly reduced as the particlescollide with ions of the opposite polarity in the gas. The resultingparticle charge can be unipolar or bipolar depending on the operatingconditions of the high-voltage corona ionizer and the electrosprayparticle generator. When the applied voltage on the electrode 152 is AC,gaseous ions of both polarities are produced. These gaseous ions wouldthen collide with the particles to form an aerosol carrying chargedparticles of both polarities. The charged particles can then bedeposited by applying an electric field on the wafer as previouslydescribed.

In other types of electrospray particle generating apparatus such asthat described in U.S. Pat. No. 5,247,842 the electrosprayed dropletsare evaporated and neutralized by bipolar ions at the same time. Thepresent invention makes use of a dry gas source 170. The electrosprayeddroplets are thoroughly evaporated in the spray chamber 130 leavingsuspended in the gas only dry solid charged particles. It is thisaerosol containing dry suspended solid particles carrying a highelectrical charge that is mixed with the ionized gas flowing through 180to have the particle charge neutralized or substantially reduced.

Aerosols generated by compressed gas atomizers are usually not of auniform size and are referred to as polydisperse. For many applications,it is desirable to use particles that are monodisperse, i.e. of auniform size. A method of classifying the particles according to sizemust be used as part of the apparatus to produce monodisperse particlesfor deposition on a wafer.

FIG. 3 shows an apparatus for producing a monodisperse aerosol byelectrostatic classification. The charged particle generator shown at180 can be either the charged particle generator illustrated in FIG. 1or in FIG. 2. The apparatus shown generally at 200 is referred to as anelectrostatic classifier, which is also known as a differential mobilityanalyzer. The apparatus 200 is of a generally cylindrical shape andcomprises a inner cylindrical electrode 214 surrounded by the cylinder212, which is in turn partially surrounded by tube 210. The spacingbetween the inner cylinder electrode 214 and the cylinder 212 and thespacing between the cylinder 212 and the rod 24 form annular gas flowpassageways, 220 and 222, respectively.

The apparatus 200 is provided with two gas inlets, 202 and 204 on topportion 205 and two gas outlets, 206 and 208 on a bottom portion 207.The aerosol carrying charged particles produced by the charged particlegenerator 180 is introduced into the apparatus 200 through the inlet202, while a clean gas that does not contain particles is introducedinto the apparatus through inlet 204. These gas streams then flow downtheir respective flow passageways in the annular space 220 and 222 nearthe top portion 205.

The cylinder 212 has a circumferential slit 224 to allow the aerosol toflow from the annular space 220 into the annular space 222. Uponentering the annular space 222, the aerosol stream joins the clean gasstream to flow down the annular space 222 in the general direction ofarrows 225. The apparatus 200 is designed such that the gas flow in theannular space 222 is laminar and there is no intermixing of the aerosoland clean gas streams. As a result, both streams flow down the annularspace 222 as laminar streams with the clean gas stream forming a cleangas sheath between the aerosol stream and the inner cylindricalelectrode 214.

The inner cylindrical electrode 214 is connected to a source of high DCvoltage while the outer tube 210 and cylinder 212 are grounded. Theelectrode 214 thus forms a high voltage electrode which is insulatedfrom the ground by insulator 216. The applied high voltage establishes aradial electric field in the annular space 222. As the aerosol flowsdown the annular space 222, the particles, which are electricallycharged, are acted upon by this radial electric field and migrate in aradial direction. If the particle charge is of the same polarity as theDC voltage on the cylindrical electrode 214, the particles are repelledby the electrode voltage and thus will migrate toward the cylinder 212and be deposited on the inner surface of the cylinder 212. In contrast,particles carrying an opposite polarity charge as the voltage on thecylindrical electrode 214 are attracted by the electrode 214 and willmigrate through the clean sheath flow toward the cylinder 214.

A slit 226 is positioned near the bottom of the cylindrical electrode214. A small amount of gas is allowed to flow through this slit and exitthe apparatus through exit 208. The gas exiting through exit 208contains particles that have migrated through the laminar gas streams inthe annular space 222 to the vicinity immediately outside the slit.These particles are of a certain electrical mobility and size dependingon the geometrical dimensions of the apparatus, the aerosol and cleangas flow rates, and the applied DC voltage on the cylindrical electrode214. For a specific DC voltage applied to the electrode 214, particleswith a smaller size and a higher electrical mobility will migratethrough the radial distance at a higher speed, and thus be collected onthe surface of the cylindrical electrode 214 above the slit. Those witha larger particle size and a lower electrical mobility will migrate at aslower speed. The larger particles with lower electrical mobility wouldbe deposited on the surface of the high voltage electrode 214 below theslit or be carried away by the main gas stream and exit the apparatusthrough the exit 206. By this means, a polydisperse aerosol introducedinto the apparatus 200 through the inlet 202 will appear as amonodisperse aerosol at the exit 208. The device thus functions as aclassifier of particle size, and will be referred to herein as anelectrostatic classifier. The electrostatic classifier thus classifies apolydisperse aerosol into a monodisperse aerosol of a narrow size rangeat the exit 208.

Particles in this monodisperse aerosol are electrically charged and canbe introduced into an apparatus for deposition on a wafer. A depositionapparatus 300 useful for the present invention includes a housing 302forming a chamber 303 in which a wafer 320 is held. Charged aerosolenters the chamber 303 through an inlet 312 in a deposition head 310. Asthe aerosol flows out of deposition nozzle 314 on the deposition head310, the aerosol impinges on the wafer surface. Both the chamber 303 andthe deposition head 310 are electrically grounded, while the wafer isheld at a high DC voltage by a high voltage DC source (not shown). TheDC voltage has a polarity that is opposite to the polarity of charge onthe particles. For instance, when an aerosol containing negativelycharged particles is used, the polarity of charge on the wafer would bepositive. By this means the charged particles are deposited on the waferwith an increased rate of deposition due to electrostatic attractionbetween the wafer and the charged particles.

The deposition head 310 can be moved in and out of the chamber 303 asindicated by arrows 305 in FIG. 3. The deposition nozzle 314 can thus bemoved along a radial direction with respect to the wafer and to aspecific radial position from the center of the wafer. Similarly, thewafer can be rotated around an axis perpendicular to the wafer by amechanism (not shown). By this means particles can be deposited atvarious angular and radial locations on the wafer. Many spots ofparticles can be deposited on a wafer by simply moving the depositionnozzle to a specific radial location and rotating the wafer to aspecific angular location, and then depositing the particles on thewafer at that location for a certain period of time until the requirednumber of particles is deposited. The method and apparatus fordeposition charged particles on a wafer by the use of an electrostaticclassifier and a movable deposition head in a chamber are described inU.S. Pat. No. 5,534,309, U.S. Pat. No. 6,607,597B2 and U.S. Pat. No.6,746,539B2 which are herein incorporated by reference.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method of depositing particles from a source onto a wafer, thewafer being disposed in a deposition chamber, the method comprising:producing an aerosol stream with solid particles suspended in a gas;ionizing the aerosol stream with a negative or positive charge polarityor both by passing the aerosol stream through a non-radioactive ionizerto produce charged particles; and supplying the aerosol streamcontaining charged particles to the deposition chamber.
 2. The method ofclaim 1 and further including classifying the ionized particles in aselected size range.
 3. The method of claim 1 wherein the particleionizer is a corona discharge type ionizer.
 4. The method of claim 1wherein the particle generator is an electrospray type generator.
 5. Themethod of claim 2 wherein the particles are classified using anelectrostatic type classifier.